Most known printing arrangements, which may be either typewriters or printers of the type which are provided at output computer terminals, utilize a cylindrical platen in combination with a plurality of smaller rollers to control the position of the paper which is to be printed. Usually, the cylindrical platen is formed of a fairly hard, yet resillient, material which may be rubber or plastic. The smaller rollers are generally formed of a softer, more resilient rubber or plastic material. The paper to be printed is arranged so that one surface of the paper contacts the cylindrical platen, which may have a diameter of approximately 2 inches, and the surface of the paper which is to be printed is in contact with the smaller rollers. Generally, the smaller rollers are arranged circumferentially about the cylindrical platen roller so as to cause the paper to be printed to be in contact with the roller platen above and below where the printing is to be performed.
It is a problem with this arrangement that the smaller rollers are required to roll over areas of the paper which have been printed. This permits the smaller rollers to pickup ink from the printed material and spread or smear the ink over the paper. In addition, the known roller arrangement tends to lose its grip on the paper. This results from the fact that the platen roller and the smaller rollers are generally made of a material which will react with the air and its pollution components to form a fairly smooth, oxidized surface thereon. Such an oxidized surface has a substantially reduced coefficient of friction which will permit shifting of the paper. As the paper is shifted slightly, a mechanical stress in the form of a shear is imparted to the paper causing it to distort. Thus, in addition to removing ink from the printed paper and smearing that ink, mechanical paper transport systems further suffer from a reduced ability to grip the paper and prevent its shifting as the material from which the rollers are formed deteriorates, and problems associated with stressing and distorting the paper. Since at least some of these distortions are produced after printing, realignment on the printer of the print on the distorted paper is practically impossible in situations where it is desired to print over the paper once again to include additional material. A particular problem arises when printing near the bottom of a page where few rollers contact the paper, thereby producing misalignment, especially when printing on card stock.
In other known arrangements for holding paper to be printed in appropriate position and orientation for printing, electrostatic forces are used to attract the paper. Generally, the paper is placed on a substantially flat surface containing buried grids on which is impressed a high voltage. Such a high voltage causes an electrostatic charge to be induced on the paper, the electrostatic charge causing attraction between the paper and the surface. Electrostatic paper holding systems are known to be used in plotters to secure the paper in a fixed position while one or more pens, or a stylus, are moved with respect to the paper to inscribe information thereon.
It is a problem with known electrostatic paper-holding systems that a residual charge may be retained by the paper. Such charges, which may be significant, may cause substantial difficulty in handling the paper after the information has been inscribed thereon and the paper has been removed from the working surface of the machine. For example, such residual charges may cause the sheets of paper to attract one another, thereby rendering stacking of the charged paper sheets difficult, particularly during periods when the ambient humidity is quite low.
It is a further problem with electrostatic paper-holding systems that the high voltages which are required to render the arrangements operable may cause radiation problems in other electronic equipment which is being used in the vicinity of the electrostatic paper-holding system. This and other difficulties with high voltage equipment are recognized by safety inspection and other certifying institutions which are charged with the responsibility of protecting the public from dangerous or unstable equipment. Thus, high voltage electrostatic paper-holding arrangements are subjected to stringent safety tests, which vary from agency to agency in the various countries. For example, in one well known certifying laboratory a voltage of up to 20,000 volts is impressed on the equipment via a hand-held wand. This voltage is applied to determine whether any ill effects are produced in the internal electrode circuitry, and such circuitry must be protected accordingly.
It is a further problem with electrostatic paper-holding arrangements that they are not reliable in holding the paper to the surface during all commonly encountered situations. For example, it is entirely probable that an operator might generate up to 15,000 volts of static electricity by walking on a carpet, particularly on a day of low humidity. The discharge of such a static charge on the electrostatic arrangement can easily unlock the paper, thereby permitting it to shift during printing. It is yet a further problem of such electrostatic systems that the surface upon which the paper is printed must be generally planar. Thus, the use of electrostatic attraction to hold the paper to a cylindrical or flexible paper transport arrangement is not feasible.
A still further arrangement which can be used for holding paper which is desired to be printed utilizes magnetic forces to achieve the desired attraction. In such an arrangement, a permanent or electrically activated magnet arrangement in the platen would attract either paper or other sheet material which is embedded with magnetic materials of opposite magnetic polarity. In addition to substantial initial expense resulting from the magnets required in the hardware, additional expense is incurred by requiring the paper stock to be embedded with magnetic material. Thus, magnetic attraction is unduly expensive.
One further system which is utilized to attract and hold paper to a surface employs a vacuum which is applied to a plurality of apertures through a surface. Although a vacuum arrangement avoids many of the problems discussed hereinabove, a vacuum hold arrangement is subject to other problems. In a vacuum arrangement where a surface having apertures is required to hold paper of various sizes, it is a problem that if the entire surface is apertured to allow freedom of placement of the paper, or accommodation of various sizes of paper, a very powerful source of low pneumatic pressure is required to maintain a sufficient vacuum force under a relatively small piece of paper, while high air volume is drawn through the remaining open apertures. This problem is particularly acute when the paper stock is both small and not very resilient, such as a 3" by 5" index card. It is a further problem with such a system that the inrush of air through the exposed apertures causes objectionable noises which may be unacceptable in a business environment. In addition, the large amount of air which is drawn through the open apertures may be accompanied by lint, dust, or other matter which can have an adverse effect on the operation of the equipment. Such problems are particularly acute when the paper is just beginning to pass a printing zone, and only a small portion of the paper is present on the vacuum platen. At such times, there may not be sufficient vacuum to hold the edge of the paper by the few apertures which engage the paper.
It is, therefore, an object of this invention to provide an improved paper and sheet material transport arrangement which is not subject to the foregoing problems.
It is a further object of this invention to provide a paper transport arrangement which utilizes vacuum to attract and hold the paper.
It is another object of this invention to provide a paper or sheet material transport arrangement which does not require special stock having special materials embedded therein to be operable.
It is still a further object of this invention to provide a vacuum operated paper transport arrangement which does not produce the objectionable wind noise of known systems.
It is still another object of this invention to provide a sheet material transport system which permits accurate alignment and realignment of a sheet to be printed upon whereby second and subsequent print cycles for adding additional text, highlighting, or graphics, will register accurately with previously printed material, notwithstanding that the sheet to be printed may have been removed from and reinstalled on the sheet material transport system.
It is yet a further object of this invention to provide a vacuum operated paper transport arrangement wherein the area over which a holding vacuum is applied is automatically adapted to the size of the sheet material being held i.e., without specific attention by the operator.